From mboxrd@z Thu Jan  1 00:00:00 1970
From: Rik van Riel <riel@redhat.com>
Subject: Re: [RFC -v2 PATCH 2/3] sched: add yield_to function
Date: Mon, 20 Dec 2010 10:40:15 -0500
Message-ID: <4D0F78DF.9010908@redhat.com>
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Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	Avi Kiviti <avi@redhat.com>,
	Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Chris Wright <chrisw@sous-sol.org>
To: Mike Galbraith <mgalbraith@suse.de>
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On 12/17/2010 02:15 AM, Mike Galbraith wrote:

> BTW, with this vruntime donation thingy, what prevents a task from
> forking off accomplices who do nothing but wait for a wakeup and
> yield_to(exploit)?
>
> Even swapping vruntimes in the same cfs_rq is dangerous as hell, because
> one party is going backward.

I just realized the answer to this question.

We only give cpu time to tasks that are runnable, but not
currently running.  That ensures one task cannot block others
from running by having time yielded to it constantly.